1. Field of the Invention
The present invention deals with a control method which incorporates the changes of the flow through the process to be controlled with determination of the control signal to the control valve or other actuator. The invention deals also with an apparatus for implementation of the method.
2. Description of the Prior Art
Control of various process quantities is implemented in industry by means of unit controllers which are commercially obtainable. When the control is implemented with devices of other type like e.g. with a computer in which the control tasks are concentrated, the control calculations generally follow known standard principles. It is characteristic of these control methods, e.g. that each process quantity under control is controlled separately and the effect of other varying quantities on the properties of the control loop and on the operation of the control is neglected. Changes of the flow through the production process disturb considerably the control of other process quantities. The flow through a normal production process is submitted to temporal changes both for random reasons, e.g. in the presence of disturbances in the same process or in a process connected serially to it, and intentionally, when the production is increased or decreased. The changes of flow may also belong to the normal operation, e.g. when the process includes devices which operate periodically like the batch digesters for cellulose pulp or when pipelines slowly get clogged under longterm use.
Because of the same changes of flow the controllers connected to the production process have to be tuned for the worst occurring case or, in practice, for the smallest occurring value of the flow. The time parameters of the process which depend on the flow are inversely related to the magnitude of the flow and, correspondingly, the bandwidth of the process, or the frequency range in which the process represents a considerable gain with regard to the input signals, is directly related to the flow. If now the controller would be tuned so that it gives an optimal result for the nominal value of the flow, and if the flow would then decrease, a narrower bandwidth of the process would result, assuming unchanged properties of the controller, whereby the control would deteriorate and the control could even fall into an oscillatory state, if the flow would become small enough. The presence of such inconvenient oscillations is known in practice i.e. in control of the temperature of a liquid by means of a tubular heat exchanger. The proportion of the time delay factor is high in such process, and this implies a negative phase shift which depends strongly on the frequency and, therefore, a tendency to oscillations.
In some cases one has tried to consider the variable flow through a compensating factor which depends on it. For example, in control of the final temperature of steam in a boiler by means of water sprayed into the steam in the middle of the superheater, the flow of the cooling water is sometimes controlled to be directly proportional to the flow of the steam. The final controlled quantity, the final temperature of the superheated steam is measured separately and the setpoint of the controller controlling said cooling water is adjusted on the basis of the measured value. Since now the speed with which the final temperature of the steam reacts to the flow of the cooling water depends strongly on the steam flow, it is seen that the described control method neglects this dependence, and the temperature controller, acting as the main controller, must be tuned for the smallest occurring steam flow.
Because of the varying flow, the continuous flow process is time variable, i.e. its parameters change with time. It has been shown theoretically that the residence time distribution of the mass flow process can be brought to an invariant form of presentation, if the continuous flow Q, which is the essential factor causing the time dependence, is taken into consideration by shifting to a new variable z (A. Niemi, Int. J. of Applied Radiation and Isotopes (1977) pp. 855-860). ##EQU1## V volume t,.nu. time variables
.eta. fixed origin of time PA1 E(s) Laplace transformation of control deviation PA1 U(s) Laplace transformation of output quantity of controller
The residence of a material in a continuous flow device can be described by means of a function of one quantity, the difference z(t,.eta.)-z(.theta.,.eta.)=z(t)-z(.theta.), even if the flow varies. If the functional form of the residence distribution of the process is known, and if the input quantity of the process is observed by means of measurements, the output quantity of the process can be calculated by means of this function, even if the flow varies.